Diffractive and refractive optical elements operate on the principle of the spatially dependent phase modulation of light. To ensure the modulation to fractions of the wavelength of the light, glass surfaces of the optical components used, for example, should have a high surface accuracy, which entails high manufacturing costs. Of considerable interest are, therefore, inexpensive phase modulators which correct the faulty phase fronts produced by simple optical components, by using a locally varying phase delay. Phase modulators of this kind, however, can, themselves, also be constructed as optical components, for example as lenses or mirrors. In this context, a spatially-dependent voltage is applied-at the phase modulator to change the focal distance of the lens or of the mirror. Another field of application of adaptive optics using phase modulators involves correcting refractive index fluctuations, as they occur, for example, in the atmosphere.
There are some recently developed deformable mirrors used for correcting large astronomical telescope mirrors. When a spatially resolved phase adaptation is made, piezoelectric, electric or magnetic positioning elements are used to tilt, shift or bend mirror sections or thin, deformable mirror surfaces. However, a phase adaptation of this kind entails heavy correction devices, and can be very cost intensive.
Another method for modulating the phase front of light provides for varying the refractive index of liquid crystals or dielectric crystals by applying a voltage; see, for example, “Principles of Adaptive Optics”, R. Tyson, Boston, 1991. When the electrooptical effect is utilized for purposes of phase modulation, an electrical voltage is applied to change the refractive index of the medium, thereby altering the optical path length and, thus, the phase of the light measured, for example, at the output of the modulator.
Besides this modulation of the so-called dynamic phase, one may also modulate the geometric or topological phase. See S. Pancharatnam in Proc. Incl. Aceal. Sei. A42, page 86, 1955. The Pancharatnam reference may show that by changing the polarization state of the light on a closed path on the Poincaré sphere, a phase is introduced in the light path. In Phys. Ref. Lett. 60, p. 1212, 1988, the R. Bhandarj et al. reference may show that a rotatable λ/4 plate in an arm of a Michelson interferometer introduces a phase difference proportional to the rotational angle between the arms of the interferometer.
The P. Harry Haran Hareharan et al. referemce of “An Achromatic-Shifter Operating on the Geomatic Phase”, Optics Communications, NL, North-Holland Publishing Co. Amsterdam, vol. 110, no. 1/02, Aug. 1, 1994, pp. 13-17, XP 000434780, may infer an achromatic phase modulator, which is constructed from a series configuration of λ/2- and λ/4 delay plates, in order to direct, in this manner, the polarization vector on the Poincaré sphere, such that different phases are compensated for various light wavelengths.
Previously available phase modulators having liquid crystals based on the topological phase are not able to provide a maximum phase shift of 360°, as is often required when a phase modulator is provided for the applications mentioned at the outset.